1. Field of the Invention
The present invention relates generally to the field of fiber optic components and, more particularly, to optical multiplexers and demultiplexers.
2. Discussion of the Prior Art
The dense wavelength division multiplexed (DWDM) optical fiber communication systems of today rely heavily upon the simultaneous launching of many optical signals, each at a respectively different wavelength, into a given fiber in order to efficiently utilize as much of the available bandwidth of that fiber as possible. The optical multiplexer/demultiplexer (Mux/Demux), also commonly referred to as a WDM or DWDM filter, is a key component of such optical systems. As will be readily appreciated by those skilled in the art, a multiplexer is used to combine optical signals of different wavelengths into a single fiber so that they may be transmitted efficiently to a remote location. Conversely, a demultiplexer separates the respective wavelength signals propagating along a single fiber so that, for example, each of the optical signals may be individually processed. Optical transmission system architects have always been concerned with maintaining an adequate system power margin in order to ensure an acceptable signal to noise ratio and low bit error rate at the desired rate of transmission. As the number of wavelengths to be added to and/or dropped from each optical fiber link has continued to increase, the need for a scalable Mux/Demux solution having low associated insertion losses has become more important than ever.
The three general types of DWDM filters principally used today are thin-film filters, arrayed waveguide gratings (AWG) and fiber Bragg gratings used in conjunction with an optical circulator. In each of these three general types of DWDM filters, optical energy or light is removed from the fiber and is either collimated into a free space light beam or redirected into a specially configured waveguide. The principal disadvantage of these DWDM filter structures is the high losses associated with moving optical energy out of the fiber and refocusing the light back into the fiber. A further disadvantage of such filters is the need for adherence to strict mechanical tolerances and the attendant vulnerability to vibration and environmental changes. In view of these disadvantages, substantial investigative effort has been directed toward the development of a filter configuration that keeps the optical energy inside the fiber. Such a configuration is generally referred to as an xe2x80x9cin-linexe2x80x9d filter.
One in-line filter approach, applicable to single-mode fiber, is described by F. Bilodeau et al. in IEEE Photonics Technology Letters, Vol. 7. pp. 388-390 (1995). The device described by Bilodeau et al. is essentially an optical channel add/drop filter based on two Bragg gratings defined in the arms of two concatenated 3 dB fused fiber couplers. The position of the two identical gratings must be accurately controlled to provide in-phase reflection in the two arms of each coupler. This interferometric arrangement requires path length trimming during device fabrication, and the precise optical phase must be maintained during the device lifetime.
Another xe2x80x9cin linexe2x80x9d add/drop filter implementation is proposed by F. Bakhti et al. in Electron. Letters, Vol. 33, pp. 803-804 (1997). The structure proposed by Bakhti et al. requires that a fiber grating be written onto the xe2x80x9cwaistxe2x80x9d or narrowly tapered portion of a fused coupler. The position of the grating related to the coupler is critical in the structure proposed by Bakhti et al. Unfortunately, the difficulty associated with realizing the precise alignment of the grating has rendered manufacture of the Bakhti et al. structure commercially impractical.
A more recent approach to the heretofore unsatisfied need for an xe2x80x9cin linexe2x80x9d filter is described by B. Ortega et al. in IEEE Journal of Lightwave Technology, Vol. 17, 1, pp. 123-128 (1999). The filter proposed by Ortega comprises a twin core fiber and single core fiber, a mixed fused tap coupler, and a fiber Bragg grating. Inside the tap coupler coupling range, there are three modes exhibited by the single core fiber, the twin core fiber""s high effective refractive index and numeric aperture (NA) core and the twin core fiber""s lower effective refractive index and NA core, respectively. According to theoretical analysis and experimental evaluation by the inventors herein, for the kind of tap coupler employed by Ortega, the high NA core mode and single core mode can be easily and efficiently coupled to each other, but the low NA core mode will be affected by the cladding mode, causing at least a three (3) db (50%) loss of optical power.
Accordingly, a continuing need exists for an in-line DWDM filter which is characterized by a repeatable, low level of insertion loss. A further need exists for a DWDM filter structure which is commercially practicable to manufacture.
The aforementioned needs are addressed, and an advance is made in the art, by an all-fiber, in line filter structure comprising a dual core fiber fused coupler and a dual core fiber grating. Essentially, the dual core fiber fused coupler of the present invention comprises at least one multiple core optical fiber and one other optical fiber. According to an especially preferred embodiment of the present invention, two dual core optical fibers are employed since these may be easily manufactured during the same production run, and under the same processing conditions, so that variations which would otherwise result in a mismatch of optical characteristics between the respective fibers sections of the coupler are substantially avoided. In this way, reliable and repeatable insertion loss performance may be readily achieved. However, provided certain fiber selection and fabrication steps are observed, it is also possible to employ a coupler having a dual core fiber fused to a single core fiber in the construction of DWDM filters according to the present invention.
Where two multiple core fiber structures are employed in the coupler, each multiple core fiber has a first core with a first effective index of refraction and a first propagation constant and a second core with a second effective index of refraction and a second propagation constant. The first and second multiple core fibers are aligned and fused together such that the second core of the first multiple core fiber is in sufficient proximity to the second core of the second multiple core fiber as to obtain overlapping mode fields and efficient coupling of propagating optical signals therebetween, while the first core of the first multiple core fiber is sufficiently separated from the first core of the second multiple core fiber as to obtain weak or substantially no coupling of propagating optical signals therebetween.
In accordance with an illustrative embodiment of the present invention, the respective first core of each corresponding multiple core fiber extends along a longitudinal, geometric central axis of the corresponding multiple core fiber, while the respective second core of each corresponding multiple core fiber extends along a longitudinal fiber axis offset from a geometric central axis of said corresponding multiple core fiber. The effective index of refraction of each respective first core is lower than an effective index of refraction of a corresponding second core.
An illustrative filter device constructed in accordance with the present invention is obtained by defining, in the second core of one of the multiple core fibers, a fiber Bragg grating. Illustratively, this may be achieved by making the second core, but not the first core, sensitive to incident ultraviolet radiation, such that the grating may be formed by photolithographic pattern definition and UV exposure. To accommodate an all optical add or drop functionality, as in DWDM multiplexers and demultiplexers, respectively, the period of the fiber Bragg grating is selected to efficiently couple an optical signal at a corresponding selected wavelength, propagating in a first direction along a first core of the first multiple core fiber, to the second core of the first multiple core fiber as an optical signal at the selected wavelength propagating in a second direction opposite to the first direction.